| Cellulose and glucose can be alcoholysed to obtain biofuels and chemicals such as 5-ethoxymethylfurfural(5-EMF),ethyl levulinate(EL),and ethyl glucoside(EDGP).As the monomeric structure of cellulose,it is important to explore the mechanism and reaction characteristics of glucose alcoholysis reaction for the largescale production of liquid fuels and high-value chemicals from cellulose.In this study,the mechanism of the alcoholysis of glucose at high concentrations(100 g L-1)catalyzed by Br(?)nsted acid was investigated using a combination of density functional theory(DFT),molecular dynamics simulation(MD)and experiments to study the alcoholysis properties of glucose in ethanol and ethanol/tetrahydrofuran(THF)at different temperatures.The distribution of glucose alcoholysis products was obtained by gas chromatography-mass spectrometry(GC-MS)analysis;the effect of THF on the energy barrier of alcoholysis reaction was obtained by quantum chemical calculations,the role of solvent system in the alcoholysis reaction was obtained by MD simulation,and the mechanism and pathway of alcoholysis of high concentration glucose catalyzed by Br(?)nsted acid were proposed.On this basis,Br(?)nsted(B)acid and Lewis(L)mixed acid catalyzed cellulose alcoholysis for biofuel EMF and EL studies were investigated.The effects of Lewis acid,H2SO4 concentration,L/B acid ratio,reaction time,reaction temperature and substrate concentration on EMF,EL yields were investigated,the possible reaction pathways of cellulose alcoholysis were obtained by GC-MS analysis,and the kinetics of cellulose alcoholysis chemical reactions were studied,and the main conclusions were obtained as follows:1)The mechanism of alcoholysis of glucose in ethanol and ethanol/THF systems catalyzed by ultra-low concentration of sulfuric acid(0.1 wt% H2SO4)at different temperatures showed that at lower temperatures(130 °C,160 °C),the system mainly performed isomerization of glucose to fructose,and the introduction of THF promoted the degradation of glucose in the ethanol system;At higher temperatures(190°C,220°C),the ethanol system mainly performed isomerization of glucose to fructose and further degradation of fructose reaction,and the main final product was EL,and the introduction of THF was detrimental to EL generation.GC-MS analysis showed that at 130°C,the main products in ethanol system and ethanol/THF were levoglucosan(LG),ethyl glucoside(EDGP)and levoglucosenone(LGO),the main products at 160℃ are furanone,LG and EDGP,and the main products at 190 and 220℃ are EL and 5-EMF.The possible reaction paths for alcoholysis of high concentrations of glucose catalyzed by Br(?)nsted acid at low and high temperatures are: low temperature:(1)G/F path for isomerization of glucose to fructose(2)G/E path for glucose to glucoside(3)G/LG/LGO path for the first generation of LG and then the decomposition of LG to LGO;high temperature:(1)G/F/EMF/EL path for continued degradation of fructose to EMF and EL(2)G/LG/LGO/EL path for continued alcoholysis of LGO to EL.2)The sulfuric acid concentration had a significant effect on the distribution of alcoholysis products.An appropriate increase in sulfuric acid concentration at 130°C was beneficial to the degradation of glucose and LGO production,but too high a sulfuric acid concentration led to the decomposition of LGO into furans;at 190°C,an appropriate increase in sulfuric acid concentration was beneficial to EL production,while too high a concentration led to an increase in humic substances.3)DFT calculations found that the energy barrier for the conversion of glucose to EDGP in the ethanol system(34.4 kcal mol-1)was higher than that in the ethanol/THF system(17.4 kcal mol-1),and THF was favorable for the alcoholysis of glucose to EDGP.MD calculations found that at low temperature,sulfuric acid was more tightly bound to glucose molecules in the ethanol and ethanol/THF systems,and THF could promote the conversion of sulfuric acid to glucose molecules;at high temperature,ethanol molecules are more tightly bound to glucose molecules,and the addition of high temperature THF is not favorable to the conversion of glucose to EL.4)Br(?)nsted acid and Lewis mixed acid-catalyzed cellulose alcoholysis showed that aluminum chloride as Lewis acid improves the selectivity of the target product and reduces the production of humic substances as by-products.The optimal process conditions for the mixed acid-catalyzed cellulose production of EMF and EL were obtained from single-factor experiments as follows: substrate 50 g/L,0.3 wt% H2SO4,reaction temperature 210 °C,reaction time 60 min,aluminum chloride/H2SO4 = 0.5(mol/mol),and the total yield of the target product was 47.2 wt%.Chemical reaction kinetics studies have found that temperature has a significant effect on the rate of product generation,with high temperatures increasing the rate of by-product generation faster than the target product,and excessive temperatures leading to by-product and humins generation;The activation energies of humic substances and by-products(glucose and fructose),EMF and EL were 104.721 k J·mol-1,53.606 k J·mol-1 and 92.061 k J·mol-1,respectively,and the activation energies required for by-products were higher than those for EMF and EL,indicating that the reaction conditions were favorable for the synthesis of EMF and EL. |
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